1. Field of the Invention
This invention relates to output regulation of an isolated power supply that uses a winding on a transformer for feedback. More specifically, it concerns a regulated flyback converter with spike suppressing coupling inductors.
2. Description of the Prior Art
A piece of electronic equipment requires a power supply unit that converts available power line voltage or battery voltage to voltage values required by the equipment. One class of power supply provides electrical isolation between the output and the input. This means there is no connection from the output of the power supply to the input. These "isolated" power supplies use a transformer as the isolation element.
Power supplies usually require regulation to control and maintain the voltage output within a specified range. A typical power supply for a computer application will have +5 volts +/-5% as an output specification. Meeting this specification requires that the power supply control the output voltage with some regulating device.
To meet the needs of both isolation and regulation, various approaches have been taken. Initially, a large transformer was connected directly to an alternating current (AC) source to step down the voltage. Then the voltage was converted to direct current (DC) with a rectifier and subsequently filtered. Regulation was provided by a device called a regulator which converts one DC voltage to another. This approach proved bulky (due to the size of the transformer for a typical 60 Hz source frequency) and did not directly address DC input applications. The regulator on the secondary side could be either a switching or linear regulator. The linear regulator drastically reduces the efficiency of the power supply.
Another approach is an isolated switching power supply that combines the efficiency of a switching regulator with the isolation of a transformer. This power supply creates AC (required to pass energy through the transformer) from a DC input. Higher frequencies (10 KHz or above) are used to reduce the size and weight of the transformer. The DC input can be obtained from an AC source with a simple rectifier and capacitor filter. Placing the transformer within the switching regulator itself tends to make regulation difficult and more costly. The complexity of the regulation scheme depends on the topology chosen for the switching regulator. Regulation requires that the output voltage is set at a specified level. The output voltage is adjusted until it matches a target or reference voltage. This is accomplished by using the well known negative feedback technique. The output is fed back and compared with a target voltage to generate an error voltage. The error voltage causes corrective action to be taken such that the error voltage is minimized. Power supplies that do not use feedback will have an output voltage that varies with the demands of the load.
The flyback topology is a low cost method for implementing isolated, switching power supplies. It includes an inherent isolation transformer that is a two winding inductor and does not require additional inductors. Energy is stored in the primary winding during the first portion of a switching cycle and released to the secondary winding during the second portion of the switching cycle. This topology has the added benefit of providing feedback information from the power transformer windings in the form of a voltage generated by transformer action during the second portion of the switching cycle.
During one switching cycle, a transistor connected to a transformer primary winding is turned on causing current to rise linearly, but no current flows through a secondary winding and a tertiary winding due to the blocking action of diodes in output circuits. The transformer is then turned off causing a voltage reversal on the primary winding (created by the magnetizing inductance of the primary). During this interval current flows through both the secondary and the tertiary windings delivering energy to the respective loads. The voltage on the tertiary is proportional to the secondary voltage by the turns ratio of the two windings (neglecting effects of leakage inductance). The tertiary output voltage can be used as a feedback voltage to regulate the output of the power supply as long as the leakage induced error terms can be neglected.
Transformer leakage inductance degrades the regulation of the flyback power supply by introducing voltage spikes into the transformer tertiary winding used for feedback. Secondary leakage inductance (parasitic series inductance inherent to any transformer winding) causes leading edge voltage spikes on the tertiary winding used for feedback. A tertiary filter will peak charge to the peak of the spike voltage (rather than the plateau) which introduces an initial error voltage into the feedback system. The peak voltage is proportional to the load current which means that the output voltage will vary greatly with changing load currents.
To solve the problem of voltage spikes in the feedback, one solution is to abandon tertiary winding sensing and close the feedback loop directly from the output voltage. Applications requiring isolation need a pulse transformer or opto-coupler to get the signal across the isolation boundary. Linear techniques can be used with optocouplers, but suffer from gain variations and are sensitive to noise. Modulation-demodulation schemes with either optocouplers or pulse transformers have less gain variation and are less sensitive to noise, but create additional circuit complexity and increased cost. Optocouplers can also cause reliability problems. Secondary sensing techniques usually require an error amplifier and voltage reference on the secondary side of the transformer and can experience difficulty during start up (initial turn on) of the power supply. This strategy results in an excessive number of parts which increases cost.
Another solution is to filter the spike out of the tertiary waveform. The time constant of a simple single pole filter can be used to eliminate the spike. However, feedback voltage itself is also filtered and such filtering of the feedback voltage slows down the response of the control loop leading to overshoot on the output or instability. The time constant of the filter determines the minimum amount of time the "plateau" feedback voltage must be present. This constraint requires a minimum load on the power supply or a reduction in power supply frequency to increase the plateau time relative to the width of the spike voltage. Load restrictions reduce the flexibility of the power supply while decreasing the switching frequency makes the power supply bigger. Filters with higher complexity can be used to improve response and performance but add additional components and cost.
A third possible solution is to operate the power supply open loop with "Feed Forward" from the input voltage. The pulse width for the transistor switch controlling the output voltage can be determined analytically knowing the line voltage and load current. This approach is complex and suffers from accuracy problems over wide ranges of line voltage, load current and temperature.
A fourth approach involves "blanking" the spike. This requires a circuit with a transistor switch that is open during the spike and closed when the plateau feedback voltage is present. The algorithm for opening and closing the switch depends heavily on the load current since both the spike and the feedback voltages are affected. The analog switch action itself can also introduce voltage spikes. This "digital filtering" technique requires either a complex algorithm or a restricted load range to be accurate.
A fifth approach is the pure sample and hold technique used in digital signal processing. The waveform is sampled during the plateau portion. A widely varying load will shift the position of the plateau voltage and change the width. This technique also requires a complex algorithm or restricted load range to accurately sample the plateau feedback voltage.